A. Field of the Invention
The invention broadly relates to broadband telecommunications systems and particularly to those employing point-to-multipoint Passive Optical Networks (PON).
B. Prior Art
Currently there are broadband service providers deploying point-to-multipoint passive optical network systems to provide voice, data, and video services to customers. There are many point-to-multipoint PON technologies available today including Broadband PON (BPON), Gigabit Ethernet PON (GEPON), and Gigabit PON (GPON). Standards bodies such as the International Telecommunication Union (ITU) and Institute of Electrical and Electronics Engineers (IEEE) have released standards for PON systems.
Systems based on point-to-multipoint passive optical network (PON), (see FIG. 1) generally comprise an Optical Line Terminal (OLT) or Optical Line Termination (OLT) connected via fiber to a 1:n passive optical splitter, which in turn is connected to a plurality of Optical Network Units (ONUs) or Optical Network Terminals (ONTs). Optical Line Terminal and Optical Network Unit is the preferred naming convention for IEEE based PON, Optical Line Termination and Optical Network Terminal is the preferred naming convention in ITU 984.x PON. This invention is independent of the specific PON technology used at the OLT and ONU/ONT. For simplicity this specification will use the term Optical Line Terminal (OLT) and Optical Network Unit (ONU) to represent the typical elements of the PON system. The OLT contains an Optical Transmitter, an optical receiver, and a Wavelength Division Multiplexer (WDM). The Optical Transmitter transmits data downstream to the ONUs on an optical wavelength. The Optical Receiver receives data upstream on an optical wavelength from the ONUs. A Wavelength Division Multiplexer (WDM) is typically used to separate the optical wavelengths.
The ONUs contain an Optical Transmitter which transmits upstream data on an optical wavelength to the OLT, and an Optical Receiver to receive downstream data on an optical wavelength from the OLT. As with the OLT, a WDM is typically used to separate the optical wavelengths. Data is broadcast downstream from the OLT and appears at all ONUs via the optical splitter. In the upstream direction, the ONUs use Time-Division-Multiple-Access (TDMA) to send upstream data to the OLT. Each ONU is assigned a timeslot to send its upstream data to the OLT. This insures that signals from multiple ONUs do not collide at the upstream port of the optical splitter. This type of ONU that operates at two wavelengths will be referred to as a two-wave length ONU.
The PON systems mentioned above operate at two wavelengths and are typically used to provide data services such as web browsing, Voice over IP (VoIP), and IP video. These services are modulated on optical wavelengths as base-band digital signals and will be referred to from now on as base-band services. In addition to these services, some PON systems also provide an RF video service that is similar to a cable TV service. In a typical scenario, this service includes several RF channels that occupy a RF frequency spectrum from 50 to 870 MHz. Some of these channels are analog video channels that use a modulation technique called Amplitude Modulated Vestigial Side Band (AM-VSB), while some channels are digital channels that use QAM (Quadrature Amplitude Modulation). This RF frequency band comprising analog and digital channels is modulated into an optical carrier at wavelength λ3 and inserted into the PON using a WDM as shown in FIG. 2. At the subscriber side, the wavelength is separated using a WDM and converted into RF for distribution within the customer premises. This type of ONU that operates at three wavelengths will be referred to as a three-wavelength ONU.
In the descriptions of various figures, we refer to a two-wavelength ONU or a three-wavelength ONU. In general, these ONUs can also be referred to as multi-wavelength capable ONUs.
The prior art three-wavelength system in FIG. 2 was an improvement over the two-wavelength system shown in FIG. 1 because it gave the system operator the ability to provide another revenue generating service. However, the system in FIG. 2 has certain limitations that prevent the System Operator from providing advanced video services such as Video on Demand (VoD) and Network Digital Video Recorder (Network DVR). These services require a set top box at the customer premises that can communicate upstream the customer's requests such as movie selection, channel selection, pause, fast forward, etc. These upstream RF signals typically occupy a frequency band from 5 to 42 MHz. When the customer activates the set top box, typically via a remote control, to request movies, or to pause a movie that is currently playing, this request is modulated into a RF carrier by the set top box and sent upstream to the set top box controller that processes the request. The system in FIG. 2 is not capable of transporting these types of upstream RF signals.
It will be appreciated that a set top box isn't the only device at the customer premises that can generate upstream RF signals. Upstream RF signals can also be generated by cable modems or other devices that offer other services.
In summary, what is needed is a system that carries upstream RF set top box and cable modem information while simultaneously supporting downstream RF video and bi-directional base-band services on the PON.